1. Field of the Invention
The present invention relates to an image display apparatus for projecting an image displayed on a display device (for example, a reflective display device) onto the eyes of a viewer as a virtual image, and to a head mounted display (hereinafter referred to as an HMD) equipped with the image display apparatus.
2. Description of the Prior Art
An apparatus is known as a so-called HMD which is mounted on the head of a viewer for projecting the image, which has been produced by a display device, as a virtual image onto the pupil of the viewer across an eyepiece optical system. The display device employed in the HMD is classified into a transmissive type illuminated from the back of its screen and a reflective type illuminated from the front side or the viewer side.
The transmissive type of the display device may be implemented by a transmissive type liquid crystal display (LCD) device which is commonly used in the HMD. As the transmissive type LCD device includes a liquid crystal panel sandwiched by a pair of substrates illuminated from the back by light of illumination, its substrate has to be high in the transmissivity of light such as a glass material. This limits the semiconductor process in the production thus making the integration of pixels at higher density difficult. Also, the transmissive type of the display device is essentially equipped with periphery circuits which includes switching elements (for example, TFT switches) for turning the transmission of light on and off at each pixel, and patterns of wiring. In addition, these periphery circuits are required to be disposed on the same display panel. When the integration of pixels is increased, the proportion of the pixel area to the screen of the display panel (so called the aperture) becomes smaller thus declining both the illumination efficiency and the quality of the image.
On the other hand, the reflective type of the display device has a substrate made of a semiconductor material such as silicon and can thus be increased in the integration of pixels while decreased in the dimensions. Moreover, this allows a periphery circuit to be mounted on the other side of the substrate opposite to the display side, thus ensuring the aperture not declined when the integration of pixels is increased. As the result, the reflection efficiency will significantly be high thus producing a brighter image. This advantage becomes emphasized as the size of pixels is reduced. While the number of pixels remains unchanged, the display panel can be scaled down without sacrificing the brightness. Alternatively, while the size of the display panel remains unchanged, the number of pixels can favorably be increased thus contributing to the higher definition of the image.
Using such a reflective type display device having a variety of merits, the HMD can be advantageous for displaying the image at higher brightness and definition.
FIG. 23 is across sectional view schematically showing an arrangement of a conventional image display device of which the display device is of a reflective type. In the arrangement, a PBS (polarized beam splitter) 103 is provided across the optical path between a light source 101 and a reflective display device 102 for separating between an optical path of illumination light emitted from the light source 101 and an optical path of view light released from the reflective display device 102. More specifically, the illumination light emitted from the light source 101 passes through the PBS 103 and is directed toward the reflective display device 102. A reflection (view light) of the illumination light reflected on the reflective display device 102 is reflected by the action of the PBS 103, passes through an eyepiece lens 104, and is directed toward the pupil EP of a viewer. This allows the viewer to view an enlarged image of the image displayed on the reflective display device 102.
The HMD using the conventional image display apparatus is used for a considerable length of time when has been mounted to the head of the viewer and its image display apparatus will hence be as small in the size and light in the weight as possible. However, the conventional image display apparatus includes the PBS 103 which is considerably heavy and will thus remain not lightened.
For compensation, an image display apparatus is disclosed in Patent Document 1, listed below, where the optical path in an illumination optical system are separated from the optical path in a view (eyepiece) optical system with no use of a PBS disposed at the intersection of the two optical paths. This will be explained below in more detail.
FIG. 24 is across sectional view schematically showing an arrangement of the image display apparatus of Patent Document 1 for use in an HMD. In the image display apparatus, the light emitted from a light source 201 is converged by a collector lens 202 and directed across a polarizer 203 to a display device 204. An image light from the display device 204 is directed toward the pupil EP of a viewer through another polarizer 205 and an optical element (prism) 206. In particular, those optical elements are arranged so that the reference axis L0 extending from the light source 201 to the display device 204 and the reference axis L0 extending from the display device 204 to the optical element 206 are intersected by each other at the location of the display device 204. This arrangement is intended to reduce the overall dimensions of the image display apparatus.
FIG. 25 is across sectional view schematically showing an arrangement of an image monitor apparatus disclosed in Patent Document 2, listed below. In this apparatus, the light emitted from a light source 301 is reflected by the reflective/transmissive surface 302a of a prism 302 and received by a displaying means 303. A reflection of the light from the displaying means 303 is received again by the prism 302 where it is passed through the reflective/transmissive surface 302a this time and then passed across a polarizer 304 and another prism 305 before directed to the monitoring point of a viewer. Since the prism 302 is shared between an illumination optical system for illuminating the displaying means 303 and a monitor optical system for directing the light from the displaying means 303 to the monitor position of the viewer, the apparatus can be decreased in the overall size and widened in the angle of view.
The HMD disclosed in Patent Document 1 is such that the display device 204 is of a ferroelectric LCD type having a wider angle of view, and the image light from this display device is directed toward the pupil of the viewer via an enlargement optical system (a prism 206). Another prism is bonded to the enlargement optical system and its bonding interface is arranged of a half mirror surface. Accordingly, the viewer can view the image received through the half mirror surface from the display device and simultaneously an external scene projected across the prisms.
In addition, the HMD in which the reflective LCD device is actuated in a field sequential mode (time-division mode) and the image light in the R, G, and B colors is directed via the hologram optical system toward the pupil of the viewer, thereby providing the viewer with the image in colors (virtual image) is disclosed, for example, Non-Patent Document 1, listed below.
The patent and non-patent documents mentioned above are as follows:                Patent Document 1: JP-A-2000-249969        Patent Document 2: JP-B-3461297        Non-Patent Document 1: “Novel Virtual Image Optics or Reflective Micro-displays” by H. Mukawa et al., SID Conference Record of the International Display Research Conference, ISSN1083-1312/00/2001-0096-$1.00+0.00, 2000 SID (a preparatory proceeding at the Conference from Sep. 25 to 28 in 2000)        
However, as disclosed in Patent Document 1, when the reference axis L0 which extends from the light source 201 to the display device 204 remains not bent but held straight, the optical path has to be lengthened for permitting the collector lens 202 to converge the illumination light emitted from the light source 201 on the screen of the display device 204. Accordingly, the apparatus will be limited in the downsizing and configured in an unfavorable layout possibly with its light source 201 projecting towards the viewer. Alternatively, when the collector lens 202 is increased in the optical power for converging the illumination light emitted from the light source 201 on the screen of the display device 204, its dimensions will increase and then prevent the apparatus from being decreased in the overall size.
Also depicted in Patent Document 1 is the optical path which extends from the light source 201 to the display device 204 is bent by the action of a mirror. The mirror is however located at the light source 201 side about the optical path which extends from the display device 204 to the optical element 206. This causes the light source 201 to be positioned far from the optical element 206 for projecting the light onto the mirror. As the result, the apparatus will be increased in the height thus interrupting the downsizing.
Patent Document 2 has similar drawbacks. Assuming that the light of illumination is directed from the light source 301 to the displaying mean 303 and the light of view travels along the optical path of the light of illumination and is directed from the displaying means 303 to the prism 305 in the view optical system, the optical path of the light of illumination is bent by the prism 302 at the light source 301 side about the optical path of the light of view. Accordingly, the optical path in the illumination optical system has to be lengthened between the light source 301 and the prism 302 to converge the illumination light on the displaying means 303. As the result, the downsizing of the apparatus will be interrupted.
It is also essential for increasing the quality of a color image (to be displayed on a display device and viewed by a viewer) to widen the color reproduction area in the image. For example, when the half mirror but not a hologram optical element is used as a combiner for the image light and the ambient light as disclosed in Patent Document 1, the color reproduction area produced on the ferroelectric LCD device being actuated in a time-division mode (with no use of color filters) may be determined by the wavelength range of the R, G, and B colors of the light emitted from the light source. On the other hand, when a hologram optical element is used as the combiner for the image light and the ambient light as disclosed in Non-Patent Document 1, the color reproduction area produced on the display device being actuated in a time-division mode is determined by a combination of the wavelength range of the R, G, and B colors of the light emitted from the light source and the diffraction wavelength range of the R, G, and B colors of the light in the hologram optical element.
However, Non-Patent Document 1 does not refer to the action of widening the color reproduction area in an image to be viewed, still less the diffraction wavelength range of the R, G, and B colors of the light in the hologram optical element in combination with the wavelength range of the R, G, and B colors of the light emitted from the light source for widening the color reproduction area.